A gas turbine engine conventionally includes a compressor for compressing ambient air to be mixed with fuel and ignited to generate hot combustion gases in a combustor. The turbine receives hot combustion gases from the combustor and extracts energy therefrom for powering the compressor and producing output power, for example for powering an electrical generator. The turbine conventionally includes one or more stages of stator nozzles or vanes, buckets or turbine blades, and annular shrouds around the turbine blades for maintaining appropriate clearances therewith. As the turbine inlet temperatures have increased to improve the efficiency of gas turbine engines, it has become necessary to provide a cooling fluid, such as air, to the turbine vanes, buckets and shrouds to maintain the temperatures of those components at levels that can be withstood by the materials thereof, to ensure a satisfactory useful life of the components. Cooling of the turbine is typically accomplished by extracting a portion of the air compressed by the compressor from the compressor and conducting the portion of the compressed air to the internal portions of the turbine, including the turbine vanes, buckets and shrouds to cool the same. Any air compressed in the compressor and not used in generating combustion gases necessarily reduces the efficiency of the turbine. Therefore, it is advantageous to minimize the amount of cooling air bled from the compressor.
Turbines are typically designed to operate at full load. However, with alternative methods of generating energy, such as wind turbines, becoming more popular, it has become important that gas turbines be able to operate efficiently at part-load conditions, to generate enough power needed to supplement the alternative methods of generating energy. In addition, when alternative methods of generating energy are not available, a turbine must be available to generate power to make up for the lost alternative method of generating energy. Similarly, days when the conditions are off design for the turbine, for example, hot days, the turbine may not be able to operate at full load. On these days it may become necessary to operate the turbine under a part load condition to keep the turbine operating within its design operating parameters.
Generally, the cooling circuits in turbines are optimized for full load operations. As a result, in part load operations the cooling circuit generally provides more cooling than necessary.